1. Field of the Invention
This invention relates to power transmission belts and, more particularly, to a toothed power transmission belt which drives/is driven by a grooved pulley either one way in a predetermined path or in a reciprocating motion.
2. Background Art
Printer carriage belts, bank note conveyor belts, card conveyor belts, and other types of conveyor belts are commonly made using a toothed construction.
The common difference for the single pitch error for the belt teeth, which is calculated by measuring the length of the pitch circle or the length of the center distance, is not controlled for the belts in the above-identified environment. Similarly, the common difference for the single pitch error of teeth on a cooperating pulley, calculated by determining the outer diameter tolerance, is not controlled.
It is known to use aramid fiber or glass fiber load carrying cords in this type of belt. One example of an aramid fiber cord is disclosed in Japanese Patent Application Laid-Open No. 54-135954. The cords therein have a twist coefficient of from 1.4 to 2.6 and a thickness of from 300 to 500 denier. It is also known to make aramid fiber load carrying cords by twisting yarns which are defined by twisting a plurality of base yams together and then applying adhesive thereto. Glass fiber ropes have also been treated with an adhesive in the prior art.
In the conventional belts, described above, the single pitch difference between the single pitch of the belt teeth and the pitch of the pulley grooves is not controlled. If the single pitch difference between the single pitch of the teeth of the belts and the single pitch of the grooves of the pulleys on which the belts are mounted is large, there may be interference between the belt teeth and pulley teeth as the belt teeth move into the pulley grooves in operation. This may produce vibration and/or variation in the belt speed. With the belt being used in a printing application, printing accuracy may be compromised. Travel accuracy may be adversely affected in conveyor systems. Variation in the travel rate may produce adverse effects in other environments in which this type of belt is used which require consistent belt speed.
Additionally, in conventional belt and pulley systems, the height of the belt teeth and the depth of the pulley grooves are normally approximately the same. Since the grooves in this type of system may be relatively deep, the rotational center line of the belt may be influenced by the interference between the belt teeth and pulleys as the belt teeth enter the pulley grooves. This condition is shown generally in FIG. 19 herein. In FIG. 19, a belt 10 is shown in operative relationship to a pulley 12. The belt 10 has teeth 14, 16, 18 which move into and out of grooves 20 on the pulley 12 as the system operates. In this system, one tooth 22 between adjacent grooves 20 on the pulley 12, moves between adjacent teeth 14, 16 and presses a portion 24 of a body 26 of the belt 10 outwardly to produce a relatively sharp bend at 24. As a result of this interference between the belt 10 and pulley 12, undesirable speed variation may result with the belt 10 in use.
It is known to use aramid fibers and glass fibers in load carrying cords in systems which require cords with a high modulus of elasticity. However, when the belts are driven by small motors, or are used as printer carriage belts, bank note conveyor belts, card conveyor belts, and the like, which are not driven with a high torque, the belts made with these fibers may not be as flexible as necessary. These belts may also become highly rigid at low temperatures. As a result, the necessary starting torque and the necessary drive torque for these systems may be undesirably high.